Oil and gas wells typically require boreholes that are drilled into the earth where zones of oil or gas will be encountered. A borehole is usually lined or cased with a heavy-weight steel pipe called casing that is secured in place in the well borehole by cement that is injected between the outer surface of the casing and the borehole. Depending upon the depth of the well, additional segments of smaller diameter casing, called liner pipe, are suspended in sequence from the pipe casing and each other to extend the casing of the well borehole. When liner pipe is so suspended, a gap is created between inner and outer surfaces of the succeeding adjoining liner pipe segments.
Once the liner pipe is suspended and set in place, heavy drilling mud in the well bore is used to contain any leak occurring between casing and liner. A liner seal is then installed between the top and bottom of adjoining segments of different diameter casing or liner pipe to pressure seal any annular joint spaces created between the upper and lower liner pipe segments. When such liner seals are placed in a wellbore casing or liner, it is important to pressure test the integrity of these seals before the heavy drilling mud is removed.
A liner top test tool is used to test the integrity of the liner seal. A typical liner top test tool will have a plurality of resilient annular seal plugs that are positioned on a back-up ring or rings and a mill-head restrained by shear pins that creates fluid passages or flow paths between the seal plug back-up rings and the mill-head.
For use the liner top test tool is positioned on a work-string and slowly lowered into the wellbore through the heavy drilling mud. The drill bit on the work-string serves as a guide until the liner top test tool comes in contact with the top of the lower liner. The mill-head is then rotated to loosen any extraneous cement or debris from the liner top and heavy mud is circulated into the work-string and out of the casing to carry and remove any loosened debris from the casing.
The work-string weight on the mill-head is then increased to shear the mill-head restraining pins. When the mill-head restraining pins are sheared, the mill-head is pushed against the seal back-up rings to squeeze the resilient seals against the top of the lower liner, block the flow paths between the seal plug back-up ring and the mill-head, and form a pressure seal between the liner top test tool and the casing. In most test procedures pressure on the seal will be from the top down.
When the liner top test tool is positioned at the top of the lower liner, the fluid force in the work-string and the casing are still in balance. The work-string and test tool are then raised a few feet above the liner top and a light fluid such as seawater is pumped into the work-string until the heavy mud is pushed to a few feet above the test tool. The pressure of the light fluid now balances the force of the heavy mud. The light fluid is then slowly bled down from work-string to a point where it is determined that the light fluid can contain well pressure and that there is no leak at the pressure seal between the liner top test tool and the casing. When such condition exists, the heavy mud is no longer needed to contain the well pressure. The liner top test tool may then be pulled up, the seals released, and the heavy mud may then be pumped out of the well.
A problem with previous liner top test tool seal back-up ring designs is the restricted fluid flow passages that are created between the test tool, with the seals and back-up rings, and the casing wall as the test tool, with the seals and back-up rings, is moved through heavy mud into or out of the casing. A restriction on the diameter of a tool is its “drift diameter”. Drift diameter is a casing term determined by formula which gives the largest diameter tool that can be passed or run through a casing of a certain size/weight. It is common for tools to have drift diameters only 0.125 inches less than the interior diameter (“ID”) of the of casing. In such a situation, only 0.062 inch gap would exist between the casing ID and back-up ring OD. This leaves a limited fluid flow passage.
The limited fluid flow passages or paths also limit the cross-sectional area provided to deliver high pressure on the seal as well as impede the mud flow needed to carry the debris out of the well. In performing such a seal test, it would be an advantage have a liner top test tool with back-up seal rings that would provide an increase in the fluid passage area between the back-up rings and the mill-head in order to produce a resulting increase in mud fluid flow rates and an increase in the seal test pressure rating.
Traditional seal back-up rings are one piece solid rings. These rings, when sized to maximum drift diameter, limit fluid flow over the back-up ring when the seal is not set. If the back-up ring is much smaller than the drift diameter the fluid flow over the ring increases but the chance of seal extrusion over the ring increases. At higher test pressures a seal can extrude over the edges of a back-up ring like oozing bubble gum.